It is well known to construct a vehicle having a plurality of groung engaging means which are utilized to drive the vehicle, some of the ground engaging means being driven at all times while other ground engaging devices being selectively driven. As an example, such a vehicle could be a tractor, a combine or a loader having four wheels, with two wheels being driven at all times while two other wheels are selectively driven only in a low speed or work range. It is furthermore known to utilize a variable displacement hydraulic transmission for the auxiliary drive.
It is desirable that when the auxiliary drive means are not being utilized, that such drive means are disengaged from the ground engaging wheels. Malm et al, U.S. Pat. No. 3,458,005 and Jennings et al, U.S. Pat. No. 3,736,732 teaches an auxiliary hydrostatic drive for the front wheels of a vehicle. A fixed displacement hydraulic motor is connected to the drive wheels by means of a fluid operated clutch. The fluid operated clutch is engaged by the fluid pressure in the hydraulic lines between the pump and the motor. Furthermore, these references teache an electric operated valve connected to the primary transmission to prevent flow in the hydrostatic drive during a high speed mode of operation.